Render a wireframe with equal thickness in Cycles?
Update

As of this commit (will be in 2.72), freestyle is now natively supported in cycles. 

This means that you can set up the freestyle lines how you would in BI, then render them straight in cycles:


Select your object and enter edit mode (Tab)
Mark everything as a freestyle edge by selecting everything (A) and pressing CtrlE> Mark Freestyle edge:


Enable Freestyle in Properties editor > Render settings:


Enable Edge marks in Properties editor > Render layers:


Render.






Original answer:

You can use two scenes, one which will render the lines in freestyle, and one which will render the scene in cycles.


Create a linked copy of the new scene by pressing the + icon in the info header and selecting Link Objects:



This way moving objects around in one scene will update them in the other scene too.
Enable freestyle in Render settings > Freestyle, and adjust the line thickness as desired.
For each object, enter edit mode (Tab), select all (A), and press CtrlE>Mark Freestyle Edge:


Disable everything except Freestyle in Properties > Render layers > Layer:


In the compositor (be aware that nodes added to the compositor are scene-specific), add a new renderlayer node and set the scene to your BI scene:


Combine with an alpha over node:




When you render, both scenes will be automatically rendered and combined.

Example .blend